The present invention relates to a magnetic resonance imaging (MRI) apparatus and a RF (radio frequency) coil employed therein. The RF coil of the present invention can be used for magnetic resonance spectroscopy (MRS) and magnetic resonance spectroscopy imaging (MRSI).
A saddle coil, a slotted tube resonator (hereinafter abbreviated to STR), and a bird cage resonator (hereinafter abbreviated to BCR) are conventionally used as a MRI RF coil.
As a method for improving the receiving sensitivity and transmitting efficiency of the above MRI RF coil, the QD (quadrature detection) method is widely used. This is a method for detecting a rotating high-frequency magnetic field with high sensitivity by generating two linear magnetic fields which cross at right angles and are shifted in phase at 90.degree. so as to generate a combined rotating magnetic field or by detecting two linear magnetic field receiving signals crossing at angles and combining them by shifting their phases at 90.degree.. Reception and transmission are electrically and electromagnetically reversible. Therefore, only reception will be described hereunder.
By the QD method, when the signal intensities from orthogonal coils are equal to each other, the sensitivity is increased ideally to 1.4 times of that by the individual linear magnetic field detection method.
In the case of an object to be inspected which has an elliptic section such as the human body, however, the load of the object to be inspected on a coil is different between the orthogonal coils and the detection sensitivity is greatly different between the orthogonal coils. In this case, there is a problem imposed that the increase in the receiving sensitivity by the QD method is limited.
A BCR with four-electrode symmetry is mentioned in Japanese Patent Application Laid-Open No. 61-95234 and Japanese Patent Application Laid-Open No. 60-132547. This coil has a BCR which comprises a pair of ring elements which are apart from each other along a common vertical axial line, a plurality of axial conductive segments for connecting these ring elements electrically to each other, and a plurality of capacitive elements which are arranged in these ring elements or axial conductive segments. An example of this coil is shown in the schematic representation in FIG. 9. Numeral 20 indicates a BCR and 24 a pickup coil. In the case of a high-pass BCR, two rings 21 and 22 are divided by 16 capacitors respectively so as to form ring elements. In this drawing, the capacitive elements are omitted. The rings are connected by 16 parallel conductors (called axial conductive segments or rungs) 23 in the vertical direction so as to form the BCR 20. In the drawing, only a part of the rung is shown. The power supply method is an inductive coupling method using, for example, the pickup coil 24. The ring diameter is, for example, 490 mm and the vertical length of the coil is 400 mm. The 2 rings and 16 conductors are copper pipes 4 mm in diameter. When each of the 32 capacitors is 48 pF in capacity, the resonance frequency for generating a uniform magnetic field in the coil is 61.22 MHz.
FIG. 10 shows an equivalent circuit of the high-pass BCR. In this drawing, the inductance of the conductor forming the coil is omitted. A1 and A2 shown in the drawing are connected to B1 and B2 respectively so as to form a circuit. Capacitors C1 and C17 and neighboring rungs thereof form a current loop which can be seen. There are 16 current loops in this example. Among them, only the 1st, 2nd, 3rd, 15th, and 16th current loops are shown in this drawing (the 4th to 14th current loops are omitted). The 16th current loop is connected to the 1st current loop. Power is supplied from terminals S1 and S2 of the pickup coil 24. The pickup coil 24 is coupled inductively to the BCR 20.
To use this BCR as a transmitting coil or a receiving coil, various decoupling arts are required. An example of the decoupling art is mentioned in Japanese Patent Application Laid-Open No. 63-175403. The conventional decoupling art is often applied to coils having a simple current path such as the surface coil or saddle coil, though it cannot be applied always to coils with a complicated shape and many current loops such as the BCR. An example of the BCR decoupling art is mentioned in Japanese Patent Application Laid-Open No. 63-171548. In this example, as shown in FIG. 11, diodes are inserted into a plurality of rungs in series and the rungs are turned on or off by dedicated control lines. There is a disadvantage in this example that to control the diodes in a batch, complicated dedicated lines are required and the coil shape is complicated. Furthermore, there is another problem that these wires disturb the uniformity of the magnetic field generated by the coil.